Metal cans are generally coated in order to prevent corrosion of the inner and outer surfaces. In recent years, for the purposes of simplifying the process, improving hygiene, preventing pollution, etc., a method in which a metal can is laminated with a thermoplastic resin film, such as a polyester film, has been performed for imparting rust resistance without using an organic solvent. That is, a thermoplastic resin film is laminated on a metal sheet of tin, tin-free steel, aluminum, or the like, and then used for food can, beverage can, or aerosol can applications in which cans are subjected to severe forming processing, such as in the case of drawn cans and thin-drawn cans. In terms of cost reduction, cans for these applications have been produced through thin-drawing and ironing under even severer processing conditions.
In the case where such severe forming processing is performed, with the thinning of the metal sheet, the resin film is also thinned. The outer surface of a food can or a beverage can is generally printed in order to improve the design features. In a can formed from a resin film-laminated metal sheet, a resin film containing white or various color pigments is laminated on the metal sheet, and the laminated sheet is used as the printing substrate in order to conceal the color of the metal sheet. In the case where such a laminated metal sheet is subjected to severe processing, the resin thickness significantly decreases, wherein the absolute quantity of the added pigment in the thickness direction decreases, resulting in a problem in that the substrate does not exhibit sufficient concealability. In the case where a large amount of pigment is previously added to the resin film with forethought of this problem, the strength of the resin film decreases. Accordingly, the resin film becomes easily chipped off or scratched during processing, and it further happens that the resin film cracks and peels off. Thus, it is difficult to improve the concealability while keeping the strength of the laminating resin film high at the same time.
For example, a method in which a biaxially drawn polyester film is laminated on a metal sheet, and the laminate is used as a material for can manufacturing, has been proposed (PTL 1). However, when forming is performed through severer processing, the resin film is chipped off or scratched, or may break in an extreme case. In addition, a method in which an undrawn polyester film is laminated on a metal sheet, and the laminate is used as a material for can manufacturing, has been proposed (PTL 3). However, undrawn films are extremely brittle and thus easily cut during film forming or handling, leading to the problem of poor productivity.
In order to solve these problems, PTL 4 proposes a biaxially drawn colored laminate film composed of a surface layer made of a high-polymerization-degree copolyester and a back layer made of a copolyester containing a high concentration of a colorant. With this film, it is certainly possible to achieve both concealability and strength of the resin film at the same time.
However, in recent years, for the purpose of reducing damage to the film in a subsequent forming processing after relaxing the stress accompanying the strain applied to the film of a laminated metal sheet, heat treatment is performed in the course of a series of can manufacturing steps. In the heat treatment, in order to balance with the enamel rater value that indicates the presence/absence of coating defects in a polyester film for laminating the can inner surface, a higher heat treatment temperature has been required. However, this causes appearance defects in the film as a new problem, and there has been a demand for its solution.